Post-translational covalent modifications of proteins through conjugation with other proteins is an important biological mechanism for regulating protein metabolism and biological activity. The most well understood post-translational modifier is ubiquitin, an 8.5 kDa protein, that is covalently attached to lysines in a target protein. Polyubiquitination of a protein targets the protein for degradation.
Ubiquitin is conjugated to its target proteins through an enzymatic cascade involving a specific E1 activating enzyme, Uba1 or Uba6, a conjugating enzyme from the family of E2s, and a ubiquitin ligase that is typically from either the RING or HECT classes of E3s (Huang et al. (2004) Oncogene 23:1958-71). Target specificity is controlled by the particular combination of E2 and E3 proteins. For example, the multi-protein ubiquitin E3 ligase Skp, cullin, F-box containing complexes (SCFs) ubiquitinate targets involved in cell-cycle progression, transcription, metabolism, and inflammation, such as the cyclin-dependent kinase (CDK) inhibitor p27Kip1 and NFκB inhibitor. IkappaB (IκB).
Other proteins that are structurally similar to ubiquitin and are referred to as ubiquitin-like proteins (UBLs) have been identified that covalently modify cellular targets using their own pathways that are parallel to that of ubiquitin. Examples of UBLs include small ubiquitin-like modifier (SUMO) and neuronal precursor cell expressed developmentally downregulated protein 8 (NEDD8). Similar to ubiquitin, UBLs are covalently attached to a lysine on a target protein via an isopeptide linkage with a C-terminal glycine of the UBL, a process that is mediated via an E1 activating enzyme, E2 conjugating enzyme, and an E3 ligase.
The covalent modification of a target protein with the UBL NEDD8 is referred to herein as neddylation. The neddylation of cullin proteins, the best understood targets of NEDD8, is necessary for SCF-mediated ubiquitination and subsequent degradation of SCF target proteins (Podust et al. (2000) Proc Natl Acad Sci USA 97:4579-4584; Read et al. (2000) Mol Cell Biol 20:2326-2333). SCFs form a subset of cullin-containing ubiquitin E3s. Members of the large family of cullin-containing E3s are called cullin-RING E3s, and contain CUL1, CUL2, Cul3, CUL4A, CUL5, CUL7, or Parc, which are regulated by covalent ligation of NEDD8. Not surprisingly, the NEDD8 pathway enzymes play an essential role in cell proliferation in organisms ranging from fission yeast to mammals (Osaka et al. (2000) EMBO J. 19:3475-3484; Tateishi et al. (2001) J Cell Biol 155:571-579). Given the importance of NEDD8 conjugation in cell growth and inflammation, further characterization of NEDD8 pathway enzymes and the domains with which they use to interact with one another is needed in order to develop therapeutics that target the neddylation pathway for the treatment of disorders such as cancer and inflammatory diseases.